.TH std::uninitialized_copy_n 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::uninitialized_copy_n \- std::uninitialized_copy_n

.SH Synopsis
   Defined in header <memory>
   template< class InputIt, class Size, class NoThrowForwardIt >

   NoThrowForwardIt uninitialized_copy_n( InputIt first, Size count,  \fB(1)\fP \fI(since C++11)\fP

                                          NoThrowForwardIt d_first );
   template< class ExecutionPolicy, class ForwardIt,

             class Size, class NoThrowForwardIt >
   NoThrowForwardIt uninitialized_copy_n( ExecutionPolicy&& policy,   \fB(2)\fP \fI(since C++17)\fP
                                          ForwardIt first, Size
   count,

                                          NoThrowForwardIt d_first );

   1) Copies count elements from a range beginning at first to an uninitialized memory
   area beginning at d_first as if by
   for (; n > 0; ++d_first, (void) ++first, --n)
        ::new (static_cast<void*>(std::addressof(*d_first)))
           typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);
   If an exception is thrown during the initialization, the objects already constructed
   are destroyed in an unspecified order.

   If d_first + [0, n) overlaps with first + [0, n), the behavior is      \fI(since C++20)\fP
   undefined.

   2) Same as \fB(1)\fP, but executed according to policy. This overload participates in
   overload resolution only if

   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.        (until
                                                                             C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                                                                             C++20)

.SH Parameters

   first              -          the beginning of the range of the elements to copy
   count              -          the number of elements to copy
   d_first            -          the beginning of the destination range
   policy             -          the execution policy to use. See execution policy for
                                 details.
.SH Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.
   -
   NoThrowForwardIt must meet the requirements of LegacyForwardIterator.
   -
   No increment, assignment, comparison, or indirection through valid instances of
   NoThrowForwardIt may throw exceptions.

.SH Return value

   Iterator to the element past the last element copied.

.SH Complexity

   Linear in count.

.SH Exceptions

   The overload with a template parameter named ExecutionPolicy reports errors as
   follows:

     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

.SH Possible implementation

template<class InputIt, class Size, class NoThrowForwardIt>
NoThrowForwardIt uninitialized_copy_n(InputIt first, Size count, NoThrowForwardIt d_first)
{
    using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
    NoThrowForwardIt current = d_first;
    try
    {
        for (; count > 0; ++first, (void) ++current, --count)
            ::new (static_cast<void*>(std::addressof(*current))) T(*first);
    }
    catch (...)
    {
        for (; d_first != current; ++d_first)
            d_first->~T();
        throw;
    }
    return current;
}

.SH Example


// Run this code

 #include <algorithm>
 #include <iostream>
 #include <memory>
 #include <string>
 #include <tuple>
 #include <vector>

 int main()
 {
     std::vector<std::string> v = {"This", "is", "an", "example"};

     std::string* p;
     std::size_t sz;
     std::tie(p, sz) = std::get_temporary_buffer<std::string>(v.size());
     sz = std::min(sz, v.size());

     std::uninitialized_copy_n(v.begin(), sz, p);

     for (std::string* i = p; i != p + sz; ++i)
     {
         std::cout << *i << ' ';
         i->~basic_string<char>();
     }
     std::cout << '\\n';

     std::return_temporary_buffer(p);
 }

.SH Possible output:

 This is an example

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to            Behavior as published              Correct behavior
                       the effect description used a for loop with
                       the iteration                                discards the value
   LWG 2133 C++98      expression ++d_first, ++first, --n, which    of one operand to
                       results                                      disable those ADLs
                       in argument-dependent lookups of operator,
   LWG 2433 C++11      this algorithm might be hijacked by          uses std::addressof
                       overloaded operator&
   LWG 3870 C++20      this algorithm might create objects on a     kept disallowed
                       const storage

.SH See also

                                copies a range of objects to an uninitialized area of
   uninitialized_copy           memory
                                \fI(function template)\fP
   ranges::uninitialized_copy_n copies a number of objects to an uninitialized area of
   (C++20)                      memory
                                (niebloid)
